THE OCEAN AND ITS WORK 205 



or to crack. The water which rises in the cracks soon freezes, and 

 when the temperature is again raised, the ice will expand so that the 

 surface will be too large for the lake in an amount equal to the width 

 of the cracks, and will either override the shores or push them up by 

 horizontal pressure. If the shores are marshy they may be ridged 

 or arched up into gentle folds. Such a push may make a ridge or wall 

 about a lake if the shores are of sand and gravel. " Walled lakes " 

 are not uncommon in Canada and in the northern United States 

 (Fig. 187). The ridging may be increased to some extent by the ice 

 driven up by the waves in the spring. 



Results of Marine Erosion 



The erosive work of the ocean is constant ; in storms the waves 

 strike with great violence, at other times more gently, but always 

 some work is being accomplished. The conspicuous work of the 

 waves is on the cliffs which border the sea. The rapidity with which 

 cliffs are worn back and the sharpness of their profile depend upon a 

 number of factors : (1) the hardness or softness of the rock, (2) the 

 presence of cracks and joints, (3) the position of the beds, (4) the 

 depth of the water, and (5) the height of the waves, the work of the 

 waves being confined to a belt extending a little above high tide and 

 slightly below low tide. 



If the water at the base of a cliff is deep, the incoming waves do 

 not break. Moreover, since no rock fragments are available for 

 battering the shore, such a wall may endure many centuries with little 

 change. On those portions of the Outer Hebrides where no gravel 

 exists, barnacles are said to be as abundant on the wave-swept cliffs 

 after a storm as before. Since seaweeds often flourish upon the 

 shores where the waves are very active, they are important in protect- 

 ing the rocks upon which they grow. 



Effect of Erosion on Different Materials. — It is evident that soft 

 chalk or glacial drift will be worn back much more rapidly than hard 

 granite. At Cape de la Heve, France, where the chalk cliffs are 300 feet 

 high, the shore is being cut back at a rate of about one yard a year, and 

 the lighthouse stationed there has been twice set back. The annual 

 . loss of these cliffs, for a distance of 142 miles, is estimated to be about 

 five and one half million cubic yards. (Wheeler.) So effective is the 

 marine erosion of some chalk cliffs that some of the streams flowing 

 over them are unable to deepen their beds with sufficient rapidity to 



